1. Field of the Invention
The present invention relates to a pattern forming method, graft pattern material, conductive pattern forming method and conductive pattern material, and more particularly to a method of forming a metallic pattern advantageous for a metallic wiring plate, print wiring substrate (FPC substrate, TAB tape and the like) and semiconductor package.
2. Description of the Related Art
Currently, the fields of FPC and TAB, in which a wiring width is narrower relative to metallic wirings in other fields, are fields in which there is an increasing demand for the application of a high-frequency current.
The FPC (Flexible Printed Circuit) is a wiring substrate in which a metallic pattern is formed on a flexible substrate and is used for a head part which repeatedly operates at a high speed in a hard disk drive or an optical pickup because of its high flexibility. Alternatively, the FPC is also used for a camera, portable telephone and the like because it can foldably incorporate an electronic circuit into a narrow space therein.
The TAB tape, which is an abbreviation for Tape Automated Bonding tape, is a wiring substrate capable of thin packaging for IC/LSI using a heat-resistant film as a base substrate. As a method of the packaging, a lead and a semiconductor chip formed on the wiring substrate are directly collectively bonded with each other by means of hot pressing. The TAB tape is superior in providing higher density wiring and therefore is used for a driver substrate for LCD/PDP and the like and a CSP interposer (rewiring substrate) for a memory, DSP device and the like.
As methods of forming the metallic pattern formed in the FPC and TAB a “subtractive method”, a “semi-additive method” and a “full-additive method” are mainly known.
The subtractive method is a method in which a photosensitive layer reactive to irradiation of active light is provided on a metallic layer formed on a substrate, the photosensitive layer is subjected to image-wise exposure, the image is developed so as to form a resist image, the metal is etched so as to form a metallic pattern, and, as a final step, the resist is stripped. In the metallic substrate used in the above-mentioned method, an adhesion property is generated by means of an anchor effect resulting from applying an unevenness process to an interface between the substrate and the metallic layer in order to adhere the substrate and the metallic layer to each other. As a result, the interface part of the finished metallic pattern is rendered uneven relative to the substrate, which unfavorably deteriorates a high-frequency characteristic when the metallic pattern is used as an electric wiring. As another problem, when the metallic substrate is formed, the substrate is subjected to the unevenness process, which requires a complicated step of treating the substrate with strong acid such as chromic acid.
In order to eliminate the above-mentioned problems, a method of minimizing the unevenness of the substrate and simplifying the processing steps for the substrate by grafting a radically-polymerizable compound on a surface of the substrate so as to modify a property of the substrate surface (as examples of which, see Japanese Patent Application Laid-Open (JP-A) No. 58-196238, and pp 1481-1494 of “Advanced Materials”, 20th edition, published in 2000) has been proposed. The metallic substrate formed in the above-mentioned method can be patterned by means of the subtractive method, but, the subtractive method has its own unique problem, that is, a so-called over-etching process in which a post-etching line width becomes thinner than a line width of a resist pattern is advantageous in order to form the metallic pattern having a fine line width by means of the subtractive method (for example, see JP-A No. 2004-31588). The reason why the over-etching process is problematic is that the formation of the fine metallic pattern directly by the over-etching process easily leads to the generation of a blurred line, a faint line, a broken line and the like, making it difficult to form a metallic pattern of 30 μm or less, which is disadvantageous for the formation of a favorable fine metallic pattern. The above-mentioned conventional method further creates a problem, from the standpoint of the environmental and pricing, because the metallic film present in any area other than the pattern portion is removed and therefore wasted in large amounts by the etching process and because the disposal of waste fluid generated by the etching process is costly.
In order to eliminate the above-mentioned problems, the metallic pattern forming method called the semi-additive method has beens proposed. The semi-additive method is a method in which a thin ground substrate layer made of Cr or the like is formed on the substrate by means of plating or the like, a resist pattern is formed on the ground metallic layer, a metallic layer made of Cu or the like is formed on the ground metallic layer other than the region where the resist pattern is formed by means of the plating, the resist pattern is removed so as to form a wiring pattern, the wiring pattern is used as a mask to etch the ground metallic layer, and a metallic pattern is formed in the region other than where the resist pattern is formed. The method does not require the etching process, and therefore, is capable of easily forming a fine line pattern of 30 μm or less. The method is also effective from the standpoints of the environment and pricing because the metal is deposited only in a desired part by means of the plating. However, there is also a problem included in this method in that it is necessary to apply the unevenness process to the substrate surface in order to generate the adhesion property between the substrate and the metallic pattern, as a result of which the interface of the finished metallic pattern becomes uneven relative to the substrate and the high-frequency characteristic thereby deteriorates when the metallic pattern is used as an electric wiring.
The metallic pattern forming method called the full-additive method has also been proposed. The full-additive method is a method in which a resist pattern is formed on the substrate, the metal is deposited other than the region where the resist pattern is formed, and the resist pattern is thereafter removed. This method, which is also an etchingless method, enables the easy formation of a fine line pattern of 30 μm or less, but shares the same problem as in the case of the semi-additive method. Therefore, a novel metallic pattern forming method capable of forming a thin line pattern, reducing unevenness in the substrate interface, and reducing the waste fluid generated by the etching process is desired.
Not only a continuous metal thin film, but also a metallic fine particle pattern formed by selectively adsorbing a metallic fine particle to a specific region is attracting an attention. As a typical example of such a method, a method of spraying a nanopaste on a pattern by means of an ink jet is known (for example, see JP-A No. 2002-299833). This method, however, also includes problems, such as that a considerable amount of time is required for the pattern forming.
The substrate used for the formation of the wiring substrate is necessarily solder-resistant and is usually required to have heat resistance against approximately 250° C. Therefore, a polyimide substrate having a superior heat resistance is generally used. However, the problems associated with the metallic pattern forming mentioned earlier remain unsolved even in using the polyimide substrate. Thus, a novel metallic pattern forming method is also desired for the formation of a metallic pattern on a polyimide substrate, which is superior in heat resistance. However, the current situation is that such a method has not yet been provided.